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Štěpite, U(AsO3OH)2·4H2O, from Jáchymov, Czech Republic: the first natural arsenate of tetravalent uranium

Published online by Cambridge University Press:  05 July 2018

J. Plášil ,
K. Fejfarová ,
J. Hloušek ,
R. Škoda ,
M. Novák ,
J. Sejkora ,
J. Čejka ,
M. Dušek ,
F. Veselovský  and
P. Ondruš
...Show all authors
J. Plášil*
Affiliation:
Institute of Physics ASCR, v.v.i., Na Slovance 2, Prague 8, 182 21, Czech Republic
K. Fejfarová
Affiliation:
Institute of Physics ASCR, v.v.i., Na Slovance 2, Prague 8, 182 21, Czech Republic
J. Hloušek
Affiliation:
U Roháčových kasáren 24, CZ-100 00, Prague 10, Czech Republic
R. Škoda
Affiliation:
Department of Geological Sciences, Faculty of Science, Masaryk University, Kotlářská 2, CZ-611 37, Brno, Czech Republic
M. Novák
Affiliation:
Department of Geological Sciences, Faculty of Science, Masaryk University, Kotlářská 2, CZ-611 37, Brno, Czech Republic
J. Sejkora
Affiliation:
Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, CZ-193 00, Prague 9, Czech Republic
J. Čejka
Affiliation:
Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, CZ-193 00, Prague 9, Czech Republic
M. Dušek
Affiliation:
Institute of Physics ASCR, v.v.i., Na Slovance 2, Prague 8, 182 21, Czech Republic
F. Veselovský
Affiliation:
Czech Geological Survey, Geologická 6, CZ-152 00, Praha 5, Czech Republic
P. Ondruš
Affiliation:
Biskupský dvůr 2, CZ-110 00, Prague 1, Czech Republic
J. Majzlan
Affiliation:
Institute of Geosciences, Friedrich-Schiller University, Burgweg 11, D-07749 Jena, Germany
*
* E-mail: [email protected]
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Abstract

Štěpite, tetragonal U(AsO3 OH)2(H2O)4 (IMA 2012-006), is the first natural arsenate of tetravalent uranium. It occurs in the Geschieber vein, Jáchymov ore district, Western Bohemia, Czech Republic, as emerald-green crystalline crusts on altered arsenic. Associated minerals include arsenolite, běhounekite, claudetite, gypsum, kaatialaite, the new mineral vysokýite (IMA 2012-067) and a partially characterized phase with the formula (H3O)+2(UO2)2(AsO4)2˙6H2O. Štěpite typically forms tabular crystals with prominent {001} and {010} faces, up to 0.6 mm in size. The crystals have a vitreous lustre and a grey to greenish grey streak. They are brittle with an uneven fracture and a very good cleavage on (001). Their Mohs hardness is about 2. Štěpite is not fluorescent in either short-wave or long-wave ultraviolet light. It is biaxial (–) with refractive indices (at 590 nm) of α = 1.636(2), β = 1.667(3), γ = 1.672(2) and 2Vobs < ~5°, anomalous greyish to pale yellow interference colours, and no pleochroism. The composition is as follows: 0.12Na2O, 50.19 UO2, 0.04SiO4, 0.09 P2O5, 0.93 As2O5, 1.95 SO3, 16.41 H2O; total 107.90 wt.%, yielding an empirical formula (based on 12 O a. p. f. u.) of (U1.01Na0.02)Σ1.03[(AsO3OH)1.82 (PO3OH)0.04(SO4)0.13(SiO4)0.01]Σ 2.00˙4H2O. Štěpite is tetragonal, crystallizing in space group I41/acd, with a = 10.9894(1), c = 32.9109(6) Å, V = 3974.5(1) Å3, Z = 16 and Dcalc = 3.90 g cm-3. The six strongest peaks in the X-ray powder-diffraction pattern [dobs in Å (I) (hkl)] are as follows: 8.190(100)(004), 7.008(43)(112), 5.475(18)(200), 4.111(16)(008), 3.395(20)(312,217), 2.1543(25)(419). The crystal structure of šteěpite has been solved from singlecrystal X-ray diffraction data by the charge-flipping method and refined to R1 = 0.0353 based on 1434 unique observed reflections, and to wR2 = 0.1488 for all 1523 unique reflections. The crystal structure of štšpite consists of sheets perpendicular to [001], made up of eight-coordinate uranium atoms and hydroxyarsenate 'tetrahedra'. The ligands surrounding the uranium atom consist of six oxygen atoms which belong to the hydroxyarsenate groups and two oxygen atoms which belong to interlayer H2 O molecules. Each UO8 polyhedron is connected to five other U polyhedra via six AsO3OH groups. Adjacent electroneutral sheets, of composition [U4+(AsO3OH)22-]0, are linked by hydrogen bonds involving H2 O molecules in the interlayers and OH groups in the sheets. The new mineral is named in honour of Josef Štěp (1863–1926), a Czech mining engineer and 'father' of the world's first radioactive spa at Jáchymov.

Keywords

štěpitenew mineraluranium(IV) bis(hydrogenarsenate) tetrahydratecrystal structurechemical compositionoxidation zoneJáchymovCzech Republic
Type
Letter
Information
Mineralogical Magazine , Volume 77 , Issue 1 , February 2013 , pp. 137 - 152
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2013

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Footnotes

†

Zdeněk Mrázek (15th February 1952–15th April 1984) was a well known Czech chemical mineralogist who worked at the Institute of Chemical Technology in Prague. Zdeněk was the first person to investigate štěpite, more than 30 years ago. He attempted to synthesize U(AsO3OH)2(H2O)4 with the aim of comparing the natural and synthetic phases. This research was terminated by his tragic and untimely death. The mineral mrázekite was named in his honour (Řídkošil et al., 1992; Effenberger et al., 1994).

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